(1) Field of the Invention
The present invention generally relates to an image forming apparatus using an electrophotographic technique, and more particularly to an image forming apparatus in which an intermediate image, transferred from an electrophotographic photoconductive medium to an intermediate transfer unit, is transferred from the intermediate transfer unit to a copy sheet so that a reproduced image is formed.
(2) Description of the Related Art
Recently, electrophotographic image forming systems which are capable of forming a multiple-color or full-color image have come into practical use. These systems include a color copier system, a color printer system and a color facsimile system.
In the image forming system of the above type, a full-color image is transferred to a copy sheet as follows. Each of developed images in secondary colors (including cyan, magenta, yellow and black) is formed on a rotary photoconductive drum with a corresponding color toner or developer. These developed images are sequentially transferred to an intermediate transfer unit by a first transfer process, so that an intermediate full-color image is formed on the intermediate transfer unit. The intermediate full-color image on the intermediate transfer unit is transferred to a copy sheet by a second transfer process so that a reproduced full-color image is formed on the copy sheet.
The color toners which remain on the surface of the intermediate transfer unit after the second transfer process are removed by a cleaning blade, so that the surface of the intermediate transfer unit is cleaned and is ready for a subsequent transfer process.
In the above-described image forming system, an endless belt member arranged between a drive roller and follower rollers under a tensile stress is used as the intermediate transfer unit. The endless belt member is in contact with the rotary photoconductive drum at one location of the endless belt member. Each of the developed images, formed with one of the color toners on the photoconductive drum, is transferred to the intermediate transfer unit to form the intermediate color image on the intermediate transfer unit.
There are two types of intermediate transfer units which are used for the image forming system. One of the two types is an intermediate transfer unit which is formed with a dielectric material, or an intermediate transfer unit having at least a toner-transferred surface which is formed with a dielectric material. The other type is an intermediate transfer unit which is formed by using a medium-resistance material.
The image forming apparatus of the present invention uses the intermediate transfer unit of the latter which is formed by using a medium-resistance material. The intermediate transfer unit of this type is used to eliminate the problem of non-transfer defect of a reproduced image on a copy sheet to which an intermediate image on the intermediate transfer unit is transferred.
There are several publications in the prior art which teach improvements related to the transfer efficiency from the photoconductive medium to the intermediate transfer unit and/or the transfer efficiency from the intermediate transfer unit to the copy sheet so as to increase the quality of the reproduced image. The above-mentioned publications are as follows.
Japanese Laid-Open Patent Application No.59-50475 teaches an intermediate transfer unit having a surface with a specific surface roughness or having an adhesion-layer surface, which is formed so as to improve the transfer efficiency.
Japanese Laid-Open Patent Application No.59-23975 teaches an intermediate transfer unit having a surface of an addition-polymer silicone rubber which is formed so as to improve the transfer efficiency.
Japanese Laid-Open Patent Application No.2-213881 teaches an intermediate transfer unit having a surface formed with a protective layer of zinc stearate, which provides a lubrication on the protective layer so as to improve the transfer efficiency.
Japanese Laid-Open Patent Application No.2-198476 teaches an intermediate transfer unit including fluororesin or silicon resin which provides a small wettability of the intermediate transfer unit so as to improve the transfer efficiency.
Japanese Laid-Open Patent Application No.5-142955 teaches that the intermediate transfer unit is electrically grounded, which prevents the interference between primary transfer electric field and secondary transfer electric field, so as to improve the transfer efficiency.
In order to provide a desired level of optical density of a reproduced image, it is necessary that the transfer toner ratio of the amount of toner (the reproduced image) on the copy sheet to the amount of toner (the intermediate image) on the intermediate transfer unit is 90% or above. The image forming apparatus of one or any combination of the above-mentioned prior art references takes only a macro measure and is directed to increasing the transfer toner ratio to 90% or above.
However, even if the transfer efficiency is above 90%, a small amount of toner may remain on the intermediate transfer unit as pinpoint spots and it may not be transferred to the copy sheet. In such a case, the reproduced image on the copy sheet has non-transfer spots in a microscopic view. The occurrence of non-transfer spots in the reproduced image will lower the quality of the reproduced image. Hereinafter, this problem is called the non-transfer defect of the reproduced image.
The image forming apparatus of any combination of the above-mentioned prior art references takes no countermeasure against the occurrence of the non-transfer spots in the reproduced image, and is not effective to eliminate the non-transfer defect in the reproduced image in the microscopic view.